This invention relates to an optical fiber made of plastics, and more particularly to an optical fiber of the step-index type having a polymethacrylate core and a vinylidene fluoride base copolymer cladding.
Optical fibers that transmit light are used in various articles and facilities including information machines, medical appliances, industrial manufacturing facilities and displays for advertising and other purposes and, above all, will be used extensively and enormously in the field of optical communications.
Optical fibers developed or proposed thus far are classified into inorganic fibers and organic fibers. In inorganic optical fibers silica and multicomponent glasses are principal materials, though in some cases a glass core is cladded with an organic polymer.
For organic optical fibers poly(methyl methacrylate) and polystyrene have been proposed as the core materials, but only the former is used in commercialized plastics optical fibers of the step-index type. This is because poly(methyl methacrylate) is superior to polystyrene in the important characteristics such as the coefficient of cubical expansion, photoelasticity constant, Abbe number and scattering loss. As the cladding material it is necessary to use a polymer of which refractive index is lower than that of the core material. Usually a fluorine-containing polymer is used as will be understood from the order of atomic refraction.
For an optical fiber using poly(methyl methacrylate) as the core material, the following items are required of the cladding material: (1) to be available at low prices, (2) to be higher than about 100.degree. C. in thermosoftening temperature, (3) to be close to the poly(methyl methacrylate) in melting temperature and melt viscosity, (4) to be compatible with poly(methyl methacrylate) and good in adhesion to the same, (5) to be obtained with little impurities, (6) to be chemically stable, (7) to be high in weatherability, (8) to be flexible, (9) to be low in crystallinity, (10) to be high in transparency and (11) to exhibit a refractive index of approximately 1.40. So far, the cladding material has been selected among fluorine-containing acrylic and methacrylic resins and vinylidene fluoride base copolymers. However, it is difficult to find a cladding polymer that fully satisfies the above listed requirements. Fluorine-containing acrylic and methacrylic resins are expensive since the monomers are expensive compounds. Moreover, many of these resins are lower than 100.degree. C. in thermosoftening temperature, and are rather poor in compatibility or mutual solubility with poly(methyl methacrylate) so that a question arises as to the adhesion at the core-cladding interface. It is expected that some vinylidene fluoride base copolymers such as the ones shown in Japanese patent applications provisional publication Nos. 51-52849 and 53-60242 will be produced at relatively low costs. Although these copolymers have balanced properties, strict limitations will have to be placed on the proportions of monomers because these copolymers are not always suited for the cladding purpose in respect of melt viscosity, melting temperature and/or crystallinity.